Time-Dependent and Time-Independent Rheological Characterization of Date Syrup

The time-dependent and time-independent rheological properties of Barhi date syrup have been investigated. Rheological measurements were performed with a rotational viscometer with parallel plate geometry. The date syrup showed thixotropic behavior and a first order exponential decay model characterized the time-dependent behavior. The rate constant of the structure breakdown was found to be a function of shear rate. The steady shear flow measurements showed that the date syrup is a non-Newtonian material fit the power law model (p <0.001). The Arrhenius model described the effect of temperature on consistency coefficient; the estimated parameters from the Arrhenius equation were used to develop a prediction rheological model for the apparent viscosity. The model accurately predicts the experimental data even when extrapolating beyond parameter estimation temperature range. The time-independent viscosity model was satisfactory for modeling date syrup despite the presence of thixotropic behavior.


Introduction
The fruit of the date palm (Phoenix dactylifera L.) is one of the most economically important fruit in the Middle East and North African countries.Dates are important for human nutrition because they are a rich source of carbohydrates, proteins, minerals, dietary fiber and some vitamins in addition to being a source of rapid energy (Jasim Ahmed, Almusallam, & Al-Hooti, 2013;Al-Hooti, Sidhu, Al-Saqer, & Al-Othman, 2002;Al-Shahib & Marshall, 2003;El-Nagga & Abd El-Tawab, 2012).Dates are mostly consumed as a whole fruit or in processed form such as date paste or date syrup.Date syrup is one of the major processed forms of the dates.Date syrup is not produced like fruit juice by pressing but produced by the addition of potable water to the dates to dissolve sugars and other components, which is then subjected to a series of purification and concentration steps.Demand for date syrup in the food industry as a natural sweetener packed with nutrients is growing, and there is much potential for new food products that contain date syrup.
Studies on date processing are very limited, Ahmed and Ramaswamy (2005) investigated the effect of temperature on viscoelastic properties and visual color degradation kinetics of dates (Lulu cultivar).Recently Gabsi, Trigui, Barrington, Helal and Taherian (2013) determined the rheological properties of date syrup from three Tunisian cultivars (Menakher, Alligue & Lemsi).The date syrups from the Tunisian cultivars were found to follow the power law with a temperature sensitive flow behavior index.Razavi, Habibi Najafi, & Alaee (2007) investigated the rheological properties of low fat sesame pastes with blended date syrup as well as fat replacement with guar gum, xanthan and starch.They found that all sesame paste/date syrup blends exhibited non-Newtonian pesudoplastic behavior at all temperature and levels of fat substitution.The power law model was the best model that fitted the shear stress and sheer rate of dates blended with sesame paste with and without fat replacer.
The rheological behavior of syrup such as molasses is an important factor that affects the efficiency of the various production and refining processes such as boiling, crystallization, separation and pumping (Leong & Yeow, 2002).Rheological properties are an important quality control parameter in food industry during production, they provide insight into the food's physical properties.This will help in understanding the underlying mechanisms of momentum and heat transfer (Kaur, Kaler, & Aamarpali, 2002).Information related to the rheological properties of date syrup is quite scares despite the growth in the date processing industry.The aim of this work is to develop a complete rheological characterization model for date syrup.

Materials
Commercially-certified organic date syrup from Barhi dates produced by the Emirates Date Factory (Al-Saad, UAE) was purchased from a local supermarket.The total soluble solids were measured with a digital refractometer at 20 o C.

Sugars and Minerals Content
Sugars in the date syrup samples were determined using High-performance liquid chromatography (HPLC) on a Waters Alliance 2695 system (Waters Corporation, Milford, MA) using a Waters Refractive Index Detector (Waters 2410) and column μ−Βondapack NH 2 (300 mm length x 3.9-mm id x 10 μm particle size, Waters 084040 ) according to AOAC method 977.20.The mineral contents were determined using inductively coupled plasma atomic emission spectrometry on a Varian ICP-OES model 710-ES (Varian-Vista-MPX; Varian, Inc., Palo Alto, CA, USA) according to the standard operating protocol.Measurements were made in triplicates

Rheological Measurements
An HR-2 Discovery Hybrid Rheometer (TA Instrument, New Castle, DE, USA) equipped with a Peltier heater was used for all the rheological measurements.Trios v3.0 software controlled the rheometer and was used for data collection and analysis.A parallel plate geometry having 40 mm diameter with a gap of 1000 µm was used for all measurements.Dynamic rheological measurements were performed at 25 o C with a strain of 2%, which was confirmed to be in the linear region by performing strain sweep testing at 10 rad/s.Measurements of yield stress were performed at 25 o C on a concentric cylinder system consisting of a 28 mm vane bob with a length of 42 mm and a cup diameter of 37 mm.The sample was carefully loaded on the cup to minimize any disturbance to the sample structure and allowed to rest for 30 minutes before taking measurements.The rheometer was set at a constant speed of 0.27 RPM and span time of 5 minutes; this system was used to record the shear stress and shear rate.The steady shear flow runs were performed at 15 o C, 25 o C, 35 o C, 45 o C and 55 o C at a shear rate of 2 up to 300 1/s.Temperature sweep testing used steady shear flow at a shear rate of 50 1/s and 150 1/s.Measurements we performed at least in duplicate, and the average value was reported.

Time-Independent Rheological Model
The shear stress (τ) and shear rate ( ) data at various temperatures were fitted to the well-known power law model given by: where k is the consistency coefficient (Pa s n ), and n is the flow behavior index (dimensionless).
The effect of temperature on consistency coefficient was modeled by the Arrhenius equation given by: where k o is a constant, ΔE is activation energy, R is the universal gas constant and T is the temperature in absolute unit (K).The parameters of Eq. 2 can be easily determined using the following linearized form of Eq. 2 The temperature-dependent of apparent viscosity for a power law fluid can be described by: where η a is the apparent viscosity (Pa.s n ), and k o is a constant representing the consistency coefficient at a reference temperature.

Time-dependent Characterization
To study time-dependency, measurements of shear stress and shear rate were carried out in ascending and descending order at 25 o C. Also, measurements of the shear stress at a constant shear rate of 100, 150, 200 and 300 (1/s) were performed at constant temperatures of 25 o C for about 350 sec.Several models have appeared in the literature to describe structure breakdown during constant shearing.These include first order stress decay with zero stress value, first order stress decay with non-zero stress value, the Weltman model or structure kinetics model (Altan, Kus, & Kaya, 2005;Razavi & Karazhiyan, 2009).In this research work based on preliminary testing of the shear stress-time data, the first order stress decay model given below was found to be more appropriate for fitting date syrup data: where τ o is the shear stress at time t=0, τ eq is the equilibrium shear stress and β is the structure breakdown rate constant.

Sugars and Minerals Content
Date syrup is an aqueous extract from date fruits that is concentrated under vacuum

Dynamic Flow Measurement
Dynamic flow measurements were performed to examine the viscoelastic behavior of the date syrup, Figure 1 shows elastic modulus (Gʹ) and loss modulus (Gʺ) as a function of frequency.Both Gʹ and Gʺ appear to be frequency dependent with nonlinear behavior with respect to frequency.However, the value of the Gʺ is much higher compared to Gʹ indicating that the date syrup behaves more as a viscous fluid than as an elastic material.The ratio of the loss modulus to storage modulus is given by tan ( ) where is the phase angle between dynamic stress and strain.A phase angle of 90 o indicates perfect viscous behavior with all shearing energy dissipated while a phase angel of 0 0 indicates perfect elastic behavior with all shearing energy stored.A material with a phase between 0 0 and 90 0 shows some viscoelastic behavior.For date syrup, the phase angle ranged from 45 to 72 0 at 25 o C, which indicates that the date syrup behaves more as a viscous material than as elastic material.
www.ccsen  Furthermore, nearly all thixotropic materials have yield stresses, and the close link between thixotropy and yield stresses have been reported by earlier studies (Mewis & Wagner, 2009).As shown earlier, the measured value of the yield stress for date syrup is very low.This illustrates that although thixotropy is present, its effects might not be of significance for date syrup.

Time-independent Rheological Model
Because the date syrup was non-Newtonian, the viscosity becomes a function of the shear rate and is better expressed as apparent viscosity.Table 3 showed that the flow behavior index at different temperatures are not significantly different (p < 0.05) therefore an average value was used and incorporated into Eq. 4 resulting in the following model for predicting apparent viscosity of date syrup.

Model Validation
Figures 5a) and 5b) show the predicted and measured viscosity as a function of temperature at a shear rate of 50 (1/s) and 150 (1/s) respectively.The model prediction fall very well within the 95% confidence interval over a wide temperature range including outside the parameter estimation temperature range of 15 to 55 o C. The time-independent model predicts viscosity very well despite the presence of thixotropy, which is not significant as explained before.This model could be a useful contribution to food industry because it may help in the design of flow systems and heat transfer operations especially for applications where date syrup is used as an ingredient.Furthermore, such a model could be used in quality control of the date syrup because viscosity is an important food quality parameter.The models have the capability to predict viscosity as a function of temperature and shear rate.

Figure 5b Figure 5 .
Figure 5b Figure 5. Predicted and experimental viscosity as a function of temperature: a) shear rate 50 1/s, b) shear rate 150 1/s

Table 1 .
(Al-Hooti et al., 2002)o Brix.The sample used in this study is 71 o Brix, and the sugar and mineral content of the date syrup sample is shown in Table 1 based on dry basis.Both sugars and minerals were close to what is reported in the literature for Barhi date syrup(Al-Hooti et al., 2002).Sugars and minerals content of Barhi date syrup

Table 3 .
Parameters of the first order stress decay model (Eq.5)